The overall objective of this proposal is to obtain an understanding of the function and mechanism of action of DR-5-P aldolase and its role in the regulation of nucleoside catabolism in both procaryotes and eucaryotes. To this end, the function and mechanism of action of DR-5-P aldolase will be studied through a comparative analysis of the structure and properties of the wild-type enzyme and a number of mutationally altered proteins isolated from DR-5-p aldolase negative mutants of Salmonella typhimurium. The regulation of this enzyme has been established in S. typhimurium and now will be studied in cell cultures derived from normal rat liver and from Novikoff rat hepatomas. DR-5-P aldolase will be purified from rat liver and antibodies made against the purified enzyme. These antibodies will then be used as probes to study the regulation of the enzyme in cell cultures. Purine nucleoside phosphorylase, an enzyme which is regulated along with DR-5-P aldolase in procaryotes, will be similarly studied in mammalian cell cultures to determine if there is any common regulation of these two enzymes in eucaryotic cells. Attempts will be made to select from the Novikoff hepatoma mutant cell lines which are deficient in DR-5-P aldolase and/or purine nucleoside phosphorylase. These mutant cell lines will then be used in cellular hybridization experiments to obtain information on the regulation of these enzymes and their role in nucleoside catabolism in mammalian cells.